The field of the invention is fault sensing and control of power distribution systems. More particularly, the preferred embodiment of the present invention is directed to reducing the time period of fault sensing and control of impedance insertion or simply circuit breaker operation interrupting power transmission, i.e. to provide for interruption of power transmission in the faulted line or lines before fault currents reach unmanageable or destructive amplitudes.
In some power distribution systems, the short circuit current capability has steadily grown to the point that existing station equipment, in particular the circuit breaker is marginal or inadequate in its fault current ratings. The cause of this trend is primarily the continuing increase in power consumption per unit area. However, the problem is compounded by the requirements of system security which have prompted stiff ties to neighboring systems and multiple parallel transmission lines within each system. There are a number of convincing arguments that the growth in short circuit capability occurs at a geometric rate. This factor combined with increased installation costs and lengthened lead times for procurement of new equipment present a strong case for current limiting devices as an alternative to the historic approach of replacement and upgrating of breakers as their ratings are surpassed.
Various devices have been utilized to limit fault current duty. These include resonant L-C links, saturable reactors and at low voltages, static breakers using force-commutated thyristors. The above devices have individual advantages and disadvantages, but all share in common the disadvantage of significant power losses while operating with normal load. In addition, these arrangements utilizing reactive elements tend to be quite bulky and introduce to the system additional problems from transient over-voltages or harmonic currents.
The insertion during the fault of a resistive element in series with a bus or feeder has certain attractive advantages, but requires rapid response and sophisticated sensing and control. In order to be effective, the device should be capable of inserting the current limiting resistor into the transmission line within about one millisecond of the occurrence of the fault. Further, owing to the extensive interconnection common within many systems, operation at transmission voltages, 138 kV and higher, is desirable.
Until recently, devices were not available which were capable of both rapid response and operation at sufficiently high voltage to make current limiting by resistor insertion in transmission lines practical. During the past year, tests have demonstrated the practicality of interrupting high voltage direct current (HVDC) lines at the 100 kilovolt (kV), 1 kiloampere (kA) level in times as short as two milliseconds. Tests on improved models of this apparatus have been carried forward to 5 kA, still retaining the 100 kV modular unit size. In addition, opening times as short as 1 millisecond have been achieved on prototype mechanical switches suitable to the current and voltage levels required. Power distribution systems providing current limiting by resistor insertion of the preferred type are disclosed in U.S. Pat. Nos. 3,611,031, 3,641,358, 3,660,723, 3,657,607, 3,777,179, 3,781,606, and Re. 27,557 having a common assignee and are incorporated herein by reference; and preferred current interrupters of the crossed-field switch tube type can be turned off without requiring a natural current zero and thereby can transfer the current into a parallel resistor are cited infra.
Resistor insertion type ac current limiting devices rated for 138 kV, 10 kA interrupt capacity and based upon the foregoing breaker are disclosed in copending application of Wolfgang Knauer and William L. Dugan, U.S. Pat. No. 3,912,975, entitled "Impedance-Increasing System and In Line Device Therefor" of the common assignee and incorporated herein by reference.
Prior methods of fault sensing, e.g. measuring rms value of the fault current, require a long sensing period including several periods of an alternating current (ac) cycle which is on the order of tens of milliseconds (ms). The prior methods are suitable for conventional circuit breakers that require at least two cycles for contact opening and arc interruption of the breaker.
In the prior method of fault sensing, the circuit breaker is tripped when the measured rms value exceeds a predetermined critical level. Obviously, sensing in a shorter time period than the time interval of an ac cycle cannot be accomplished directly and the rms value must be inferred by projection of an initial fault current build-up. A further complication in this procedure of projection is that faults can start at any time within the ac cycle.
Recent developments in electrical power breakers include one-cycle and synchronous breakers and also current limiting breakers. These recently developed breakers are dependent upon very short time intervals for fault sensing to utilize their improved capabilities, i.e. sensing time intervals of less than a millisecond to a few milliseconds.
The control system is directed to a system which senses power system conditions and when a power fault is detected in an aligned sample signal related to the aligned divisions a network simulator is activated to generate a simulated fault condition in phase with the line fault. The simulated fault current provides a reference and both simulated reference fault current and sampled fault current are supplied to a differential comparator for comparison and response when appropriate.
Accordingly, an object of the present invention is to provide for improved control of ac power distribution for interrupting faulted power lines.
Another object is the provision of method and apparatus for rapid fault current sensing and control of fast-acting circuit breaker.
A further object is to provide for improved simulation of fault conditions in power transmission.
Other objects and features of the invention will become apparant to those skilled in the art as disclosure is made in the following detailed description of preferred embodiments of the invention.